This invention relate to a power supply apparatus of which operation can be stopped when the temperature of the apparatus excessively rises.
Various types of power supply apparatuses are designed for various purposes. For example, there is a power supply apparatus designed for use with welders, cutters or battery chargers. In such power supply apparatuses, for example, a commercial AC voltage is converted into a DC voltage in an input-side AC-to-DC converter, and the resulting DC voltage is converted into a high-frequency voltage in a DC-to-high-frequency converter. The resulting high-frequency voltage is then converted back into a DC voltage in an output-side high-frequency-to-DC converter, and the resulting DC voltage is applied through output terminals to a load.
The AC-to-DC converter, the DC-to-high-frequency converter, and the high-frequency-to-DC converter may be placed in a casing. Output terminals are formed to extend through a portion of a wall of the casing. The high-frequency-to-DC converter is coupled to an external load via these output terminals.
Such casing may be molded from a plastic material in order to make the power supply apparatus small and light. The output terminals of the apparatus mounted on such a plastic casing are connected to wires for connecting the apparatus to the external load. Contact resistances may be developed between the wires and the output terminals. When a large load current flows through the contact resistances, heat is generated at the contact resistances, which raises the temperatures of the output terminals.
The output terminals are also connected to the high-frequency-to-DC converter within the casing through wires. Contact resistances are present also between these wires and the output terminals. The large load current flows also through these contact resistances, which raises the temperature of the output terminals, too, for the same reason as stated above.
A utilization ratio is predetermined for a power supply apparatus for use in welding. The xe2x80x9cutilization ratioxe2x80x9d referred to herein is a ratio in percent of a time length for which the power supply apparatus is operated to a predetermined time length, for example, 10 minutes. Usually, the utilization ratio is, for example, 20% or 15%. This utilization ratio is determined by taking lightness, movableness and economy of production into account.
When the utilization ratio of the power supply apparatus exceeds a prescribed value, the casing, made of plastic, may be deformed when the temperature of the portion of the casing where the output terminals are mounted rises due to temperature rise of the output terminals caused by the above-described reasons. Usually, the output terminals have their outer surfaces threaded, and they are put into holes extending through the wall of the casing. Nuts are screwed onto the threaded output terminals to secure them to the casing. Accordingly, if the casing is deformed due to heat, the output terminals cannot be properly secured to the casing. It is, therefore, necessary that an operator of the power supply apparatus should be careful for the utilization ratio not to be exceeded, which may be a stress to the operator.
An object of the present invention is to provide a power supply apparatus which includes output terminals mounted on a plastic portion of a casing of the power supply apparatus, in which reliable securing of the output terminals to the casing can be maintained even when the temperature of the output terminals rises.
A power supply apparatus according to the present invention includes AC-to-DC converting means for converting a voltage from an AC power source to a DC voltage. The AC-to-DC converting means may include, for example, rectifying means for rectifying the AC voltage, and smoothing means for smoothing the output of the rectifying means. An appropriate one of various AC power sources may be used. For example, a single-phase AC power source or a three-phase AC power source may be used.
DC-to-high-frequency converting means, which may be, for example, an inverter or a chopper, converts the DC voltage outputted from the AC-to-DC converting means to a high-frequency voltage. Voltage-transforming means may be additionally used to voltage-transform the high-frequency voltage. High-frequency-to-DC converting means is provided for converting the high-frequency voltage from the DC-to-high-frequency converting means to a DC voltage for application to output terminals of the power supply apparatus. The high-frequency-to-DC converting means may include rectifying means for rectifying the high-frequency voltage and smoothing means for smoothing the output of the rectifying means.
The AC-to-DC converting means, the DC-to-high-frequency converting means and the high-frequency-to-DC converting means are housed in a casing. The casing includes a plastic panel which deforms when its temperature rises above its heat deformation temperature. The entire casing may be formed of a heat-deformable plastic.
The output terminals are arranged on the plastic panel of the casing. Temperature sensing means is mounted on the casing. The temperature sensing means may be disposed at any location, but it is desirable to dispose it in proximity to the output terminals. From the esthetical view point, it is desirable to position the temperature sensing means within the casing.
The temperature sensing means may be adapted to output a temperature-indicative signal indicative of the temperature around it. Alternatively, the temperature sensing means may be adapted to provide an output signal when the temperature of the casing has risen to a temperature above a predetermined temperature which is set at a value lower than the heat deformation temperature, and to cause such output signal to disappear when the temperature of the casing has fallen below the predetermined temperature.
Output control means is disposed in association with the temperature sensing means. The output control means operates to prevent the output of the apparatus from appearing at the output terminals when the temperature sensing means senses that the temperature of the casing has risen to a temperature above the predetermined temperature, and to cause the output of the apparatus to be developed at the output terminals when the temperature sensing means senses that the temperature of the casing has fallen below the predetermined temperature.
Other than the one described later, various configurations can be adopted for the output control means. For example, the output control means may be switched between a state in which the commercial AC power source is coupled to the AC-to-DC converting means and a state in which the commercial AC power supply is decoupled from the AC-to-DC converting means.
With the above-described arrangement, when the temperature of the casing rises above the predetermined temperature, the output control means operates to prevent the power supply apparatus output from being developed at the output terminals, so that the temperature of the casing does not rise any more. This can prevent the heat deformation of the panel. Further, since no output is developed at the output terminals, the temperature of the output terminals and, hence, the temperature of the casing gradually decreases, and when the temperature of the casing decreases below the predetermined temperature, the power supply apparatus output is developed again at the output terminals.
It is possible to cause no output to be developed at the output terminals by means of a temperature-sensitive fuse, which melts when the temperature of the output terminals rises above a predetermined temperature.
However, in such arrangement, the molten fuse must be replaced with a new fuse in order to make the power supply apparatus operate again. The replacement of fuses is troublesome. In contrast, according to the present invention, the operable condition of the power supply apparatus is automatically recovered when the temperature of the casing decreases.
The DC-to-high-frequency converting means may include at least one semiconductor switching means which is rendered operative when a control signal is applied thereto from means for controlling the DC-to-high-frequency converting means and continues to be conductive as long as the control signal is present. In this case, the output control means may be interposed between the means for controlling the DC-to-high-frequency converting means and the semiconductor switching means Then, the output control means operates to prevent the control signal from being supplied to the semiconductor switching means when the temperature sensing means senses that the temperature of the casing has risen above the predetermined temperature. When the temperature sensing means senses that the temperature of the casing has fallen below the predetermined temperature, the output control means permits the semiconductor switching means control signal to be supplied to the semiconductor switching means.
With this arrangement, the power supply apparatus is made to provide and remove an output to and from the output terminals by making the output control means operate to couple and decouple the control signal to and from the DC-to-high-frequency converting means. Therefore, the output control means need not interrupt a large current or voltage directly. Accordingly, the output control means need not have large current capacity and, therefore, can be of a simple arrangement.
The temperature sensing means may be disposed on the panel in the vicinity of the output terminals so that the temperature sensed by the sensor can be very close to that of the output terminals, and, therefore, the heat deformation of the panel can be more precisely prevented.
The temperature sensing means may be attached to the output terminals of the apparatus. With this arrangement, since the temperature of the output terminals which has risen above a predetermined temperature cannot decrease rapidly, the interval during which no output is provided at the output terminals can be relatively long.